John is 24 years younger than his dad. The sum of their ages is 68 years. How many years old is John?
Explanation: Let $j$ be John's age and $d$ be his dad's age. We are trying to find the value of $j$. We can create a system of two equations to represent the given information. They are

\begin{align*}
j &= d - 24 \\
j + d &= 68 \\
\end{align*}We want to find $j$, so we need to eliminate $d$ from the equations above. Rewriting the first equation we get $d = j+24$. Substituting this into the second equation to eliminate $d$, we have $j+(j+24)=68$, or $j=22$. Thus, John is $\boxed{22}$ years old.